[R] nested design

[Richard M. Heiberger]

Your model
model <- aov(Biomass ~ Beech + Age + Error(Age/Stand))

has a redundancy that might be causing the problem.  I can't tell 
without the data.

Try
tree.aov <- aov(Biomass ~ Beech + Age + Error(Stand %in% Age))


A second potential problem is the class of the variables.
>From the degrees of freedom, it looks like Stand is a factor.
Since there is only one df for each of Beech and Age, they might
be incorrectly interpreted as continuous variables instead of factors.
My guess is that they should be multi-degree-of-freedom factors and aren't.

Rich